texts:
When reading through articles about discovering gravitational waves, I got basic understanding of events that happened. But one thing wasn't clear for me. They say that two black holes collided 1.3 billion years ago.

My question is, how can we know, that this collision happened 1.3 billion years ago?


texts: Would the existence of Planet Nine rule out the possibility of a sixth giant planet?

texts:
I'm aware that there have been attempts to simulate the evolution of the Solar System with six giant planets1, as opposed to the traditional four or five. The recent proposed Planet Nine would probably invalidate some of those models, although I don't know how drastic its effects would be.

Would Planet Nine fit into models with six giant planets? Have any simulations been done that support such a  scenario?

1 The models I'm most familiar with were created by Nesvorny & Morbidelli (2012).


texts: Do black holes form by when the massive star dies and all the gravity falls in on itself, or something else. Please let me know, I'm really desperate!!


texts: Eclipse comparison: May 20 2012 vs Aug 21 2017 as observed from San Francisco, CA

texts:
I'm preparing for the solar eclipse coming in August of this year, and want to make a decision on whether it will be worth my while to travel north from San Francisco.

As observed from San Francisco, when compared to the May 2012 eclipse, will there be more of the sun obscured by the moon in 2017, or less?


texts:
Is it possible that light itself is dark matter?  I am speaking of photons (e.g. visible light, infrared, ultraviolet, etc...).  I realize light is understood to be massless, but it is obvious it at least contains energy because we can see with it (e.g. it energizes the cells in our retinas).  I wonder if light has a very tiny "net mass" (e.g. 0 mass * relativistically infinite speed).  I would think that light at least has a little mass, in proportion to its energy.  For example, take E=mc^2, then m = E/c^2 would describe how much mass it has.  If this is true, light should have a very little gravity too.  Although the effect would seem minimal, light is practically everywhere. Gravity from light would be more concentrated inside galaxies, and even more concentrated in the center of galaxies where there are many stars (like dark matter is).  It would be interesting to run the calculations, assuming light does have gravity, and see if this matches the gravitational observations of dark matter in the universe.  It would be funny and ironic if dark matter really is light.

Edit: Note that it appears light does have gravity as per the discussions here: How does light affect the universe?  If that much is true, I wonder if this is significant enough to account for dark matter?


texts: What is the distance from Alpha Centauri to Barnard's Star?

texts: Alpha Centauri AB is the closest star system to Earth (4.366 ly), followed closely by Barnard's star (5.988 ly). The closest star system to Alpha Centauri is Luhman 16 (3.8 ly from α Cen). So I am wondering, what is the distance from α Centauri AB to Barnard's star, and more generally, from one star to a different one (all below 10 pc)?


texts:
How do we calculate the escape velocity of galaxies?
Do we have to know the total mass?
What is the escape velocity for Milky Way?


texts: Why do most of the myths behind constellations originate from Greek?

texts:
Why do most of the myths regarding constellations happen to have begun from Greek civilization? Why not other civilizations?

Most people who lived in the past era probably observed star patterns in the sky and created their own constellation myths.

Also, I would like to know more about some of the stories regarding the myths of those constellations.


texts: Moons with curlicue paths around our Sun?

texts:
I naively believed that since our Moon orbits Earth, and since Earth orbits the Sun, the path our Moon might take around the Sun would be this type of epitrochoid curve:

I was surprised and delighted to find that our Moon's orbit around the Sun actually forms a convex curve which is really darn close to Earth's orbital path: Moon's orbit around the Sun.  Since the Earth's instantaneous velocity around the Sun is much greater than the Moon's instantaneous velocity around the Earth, we don't get any of the closed "curlicues" I expected to see.
Are there any moons in our solar system with epitrochoid paths around the sun that appear to have closed curlicues?  If so, what are they and what do the paths look like?


texts: What happens to a star after it has become a red giant?

texts: When our sun reaches the end of its lifetime , it will turn into a red giant. How long will it be a red giant, and what happens after a star has been a red giant? Does it turn into a planet after a long enough time spent, when it has used up all its helium?


texts: Does the age of the universe take into account General Relativity / Special Relativity?

texts: It is generally accepted that the age of the universe is approximately 12-15 billion years old based on the speed of the expansion of the universe. Since everything is moving very fast away from us, doesn't relativistic effects from high speeds come into play here? If something is moving quite fast, time slows down, affecting how "old" it is. Is all this taken into account when calculating the age of the universe? I don't know if I'm making any sense to anyone else or not.


texts: What instruments are used to measure the distance to the Sun?

texts: In the 18th century, the distance to the Sun was measured using the transits of Venus. Nowadays, the timing of radar pulses is used to measure the distance of things in our solar system. What instruments are used to perform these measurement? What do they look like (bonus points for pictures)? Are there observatories with "radar guns" dedicated to measuring the distance of the Sun?


texts:
From what I understand stars form in dense clouds of hydrogen and helium molecules. Only so much hydrogen and helium were "created" from the Big Bang or rather formed if you may.

Is it conceivably possible for the universe to not have any more hydrogen and helium to condense into a molecular cloud and form new stars? Meaning will star formation cease at some point in the far far far away future.


texts: Explain to a 4-year old why people below the equator are not tilted or suspended upside down

texts:
So I just bought a globe for my extremely inquisitive soon to be 4-year old son. He knows the very basics of the solar system and knows that we live on planet Earth and also knows the concept of gravity.

While i was explaining to him what the illuminated equator line is he asked me How do people stand below this line? (Look at the globe from a 4 year old's eyes and you will wonder the same)

I explained to him that earth is not as small of a sphere as this globe depicts and almost everywhere it will be relatively flat but then he said there are so many places on this globe at the bottom if you live there earth will be above you and gravity will hold you upside down.

How do I explain it to him?


texts: Is the halo virial mass based on only dark matter mass or also include baryonic mass?

texts:
Dark matter-only N-body simulations are ubiquitous and there exist many algorithms/codes to identify and measure the properties of dark matter halos in these simulations. One basic quantity describing dark matter halos is the virial mass (which itself can be defined in many ways, usually by finding the radius within which the mass density equals some number times the current mean cosmological density).
My question is: how can we trust the virial mass (or radius) measured in dark matter-only simulations, when they do not account for the presence of baryons? In principle, the virial mass should be the total gravitating halo mass: DM+baryons. Wouldn't we expect the radius within which the density equals some pre-defined number to change if we also had baryons distributed around dark matter overdensities? How do "halo finding" codes get around this problem when run on dark matter-only simulations?
More practically: is the "virial mass" of halos reported by halo finding codes only the dark matter mass, or the "total" (DM+baryons) mass? If the latter, I guess these codes have to assume something under the hood -- for example that every dark matter particle in the N-body simulation has associated with it the universal baryon fraction $f_b\sim15\%$.


texts: Where can I learn these Astrophysical techniques? [Read below]

texts:
Whenever I read any Astro papers in http://arxiv.org/ they usually talk about Data fitting or modelling or sentences like 'we fitted the spectral region by analytic functions  i.e. by multi-component Gaussian model plus continuum'.

Is there any websites, online books or maybe YouTube videos which teaches these techniques in detail?

Any info will be much appreciated!


texts:
I read that Great Pyramid was built to resonate with Sirius by aligning an airshaft that connected to queen's chamber. That is, the light of Sirius going through the shaft would be able to shine into the queen's chamber.

When exactly star Sirius shines into the queen's chamber of Great Pyramid?


texts:
I'm reading about the history and making of ESO's Very Large Telescope and I've found this article that says the scientific community had a choice between three suggestions: a one-piece 16m-telescope, four 8m-telescopes or 16 4m-telescopes. They chose the second solution (four 8m-telescopes), because

  there was a clear trend that some form of limited array was the right direction for the VLT.

I don't understand what limited array means. Is it just a name they picked up from radio interferometry, or is it some special method of interferometry being used at the VLT? The article says, that

  the study group [for the VLT] concentrated its efforts on the scientific advantages one could draw from the limited array concept and particularly on interferometry."

Could anyone clear this up for me?


texts: On the Equator, why are the shortest shadows seen at 11:32, and not at Noon?

texts:
On Equator, why do objects cast the shortest shadows at 11:32 and not 12:00?

For instance, this happens in Costa Rica.


texts:
Siding Spring now passes about 10 Earth diameters away from Mars.

What are the estimates of how often comets fly by Earth at different distances? Are the distances to some famous historical comets known?


texts: Relation between molecular cloud mass and galactic mass

texts: Would it be true to assume that the mass of a galaxy is approximately the mass of all molecular clouds detected within it, let's say, through CO observations and then converted to H$_2$?


texts: Easy interactive visualization of orbital elements

texts: To get a grip on orbital elements, I'd like to play with a ready interactive visualization like in Universe Sandbox. E.g. drag the inclination slider and see immediately what happens to the satellite orbit. Are there any free tools capable of doing this? I wasn't able to find such a function in Celestia or Stellarium so far.


texts: How do two supermassive black holes reach "the last parsec" in merging galaxies?

texts:
This answer to Why can't supermassive black holes merge? (or can they?) describes the barrier to merging that two supermassive black holes face when two galaxies are in the process of merging or have "successfully" merged.

Here is a bit of the answer, but its worth taking a moment to go there and read the whole thing:

  When two galaxies merge, their supermassive black holes both have angular momentum. Through a phenomenon known as "dynamical friction," gravitational interactions with other stars sap the black holes of much of their angular momentum, until they are brought within a few parsecs or so of each other. At this point, the black holes have flung out all of the stars that were in the region and there is (presumably) nothing left for dynamical friction to sap their angular momentum.

What kind of timescale are we talking about to reach "the last (few) parsec(s)? Is it similar to the time it takes for the merged galaxy to reach some kind of centralized shape (things rotating around a common center rather than having two identifiable centers of rotation)? Much longer or shorter?
What kinds of processes are involved that allow the two black holes to reach the last few parsecs of separation? What is "dynamical friction" and "star-flinging"?


texts:
Earth undoubtly has very good conditions for supporting life. Although it is expected that many other planets on the outer space have conditions at least as good as Earth, the vast majority doesn't, making them unhospitable to life or probably being able to support only very simple lifeforms. Earth itself for some billions of years until the Ediacaran or Cambrian could only support very simple lifeforms.

There are many parameters that may influence the habitability of a planet and its ability to support complex life: Star type; star temperature; star luminosity; stellar activity; stellar stability; star age; planet age; planet composition; planet size; orbital excentricity; orbital length; rotation axis inclination; planet tectonics; planet magnetosphere; presence and influence of satellites; abundance of water; planet atmosphere; interactions with other planets; presence or absence of asteroids, comets and minor planets planets belts and their position, distribution and composition; galactic orbit; galactic neighborhood; mass-extinction events rate, probability and intensity; and hundred of other possible variables including some based on pure luck and random chance.

Many of the parameters are modeled after Earth itself, since Earth is the only place so far that we know that life exists, and even if we found some alien life somewhere, it will probably be limited only to very simple forms of life.

But, what combinations of those parameters could lead to a planet with better life support than Earth itself?

Ok, you may argue that the question is too broad, so by "good life support" we could say something that allows the planet to evolve plenty biodiverse multicellular life ranging from simple microscopic creatures to complex dozens-meters long creatures with many body-differentiated parts and organs in a short timespan. So, a planet that has an environment which allows the development of richly-diverse and complex plant-like and animal-like creatures in a billion years after formation and stay like this for another 10 billion years is expected to be more life-friendly than Earth.

Further, lets restrict the biochemistry to what we know: water-based and carbon-based life, but not necessarily oxygen-breathing.

By the way, I am not asking anything about intelligent life or humans, just complex multicellular and biodiverse life.

By the way: two important links about the subject: http://www.nature.com/scientificamerican/journal/v312/n1/box/scientificamerican0115-32_BX1.html and http://www.nature.com/scientificamerican/journal/v312/n1/full/scientificamerican0115-32.html


texts: What do gravitational waves allow us to understand?

texts:
Before reading this question, you should read this paper.
From what I understood, we managed to detect gravitational waves for the first time.
Is this discovery going to help us understand something we haven't understood yet? Dark matter? Galaxy movements? FTL travel? Gravity manipulation, Interstellar style?
Though I understand the discovery, I'm a little confused about what it will actually improve for science.
So, what are the doors this discovery will open ?


texts: Are there any works discussing planetary bodies being forms of life?

texts: I'm searching for works or individuals that consider and discuss the idea of planetary bodies being considered stand alone living creatures, rather than objects either capable or incapable of supporting life.  Anyone familiar with something along those lines?


texts:
I am thinking about buying a telescope, but here in Brazil they are very expensive and hard to find where to buy. So, the better telescope I can pay for is a Celestron Powerseeker 60AZ. My main application would be to see nebulas and some star clusters, but I don't know if this scope can be used for this pourpose. Will I see big blurry images, faint lights (like a tiny fog on the darkness of the sky) or only its main stars?

*Note: I've found some images about planets in small scopes on YT, but I haven't found about DSOs (although, I know the images won't be so atractive like bigger telescope ones). Also, can I use the star diagonal as a barlow lens?

Specifications:

Optical Design: Refractor

Aperture:   60mm (2.36")

Focal Length:   700mm (28")

Focal Ratio:    12

Focal Length of Eyepiece 1: 20mm (0.79")

Magnification of Eyepiece 1:    35x

Focal Length of Eyepiece 2: 4mm (0.16")

Magnification of Eyepiece 2:    175x

Finderscope:    5x24

Star Diagonal:  1.25" Erect Image Diagonal

Highest Useful Magnification:   142x

Lowest Useful Magnification:    8.57x

Limiting Stellar Magnitude: 11.4

Resolution (Rayleigh):  2.32 arc seconds

Resolution (Dawes): 1.93 arc seconds

Optical Coatings:   Fully-Coated

Optical Tube Length:    711mm (28")

Barlow Lens:    3x


texts:
What is the light source in the black hole images?

I've searched the site before asking, as well as tried to google it. But perhaps the answer is buried due to the recent news.

My understanding, is that the ring in the images is an illuminated accretion disc.

What I could not find explained is the light source, and by extension, why is a bright accretion disc – which is round by definition – a proof for light bending? (I'm not disputing the fact.)


texts: How does one use SPICE to calculate sunrise and sunset?

texts:
I've seen it mentioned here that SPICE can be used to calculate sunrise and sunset. How?
According to the official documentation, SPICE is used to calculate:

Time system conversions
Positions of spacecraft and natural bodies
Orientation of spacecraft and instruments
Reference frame transformations
Illumination angles

I know sunrise and sunset can be calculated from the Sun's coordinates and the sunrise equation.
I was wondering if SPICE had a specific algorithm for this purpose?


texts: How many satellites orbit the Earth? At what rate do they orbit the earth?

texts:
I was looking at a full moon the other night with my telescope and noticed a black dot move across the moon quite rapidly. This happened a few times and others who were with me were able to confirm.

There is no way the ISS would make its orbit around the Earth that quickly for us to be able to see it a few times. Therefore there must have been others. How many are there, and how long does the average satellite take to make a full orbit around the earth?

The dots we saw looked much like this:


texts: How would a celestial map for Callisto depict the paths of Jupiter, the Sun, etc.?

texts:
Let's say we're mapping Callisto's northern celestial hemisphere in a way similar to this map of Earth's.

Along with the stars (which let's just ignore for this question), this map will identify the paths of four objects:

The Sun
Jupiter
Ganymede (closer to Jupiter)
Himalia (farther from Jupiter)

Where on the map would these objects' paths be? Or at least how would I find the exact answers with math/software/etc.?

My assumptions/guesses:

Jupiter's path is relatively similar to the ecliptic but Callisto's orbital inclination or maybe Jupiter's might separate the two
Jupiter's moons are within a similar orbital plane to Callisto and would follow near Jupiter's path
the distance Callisto travels away from the sun in its own orbit is insignificant in determining the shape of the ecliptic

I have no idea if the axial tilt of Callisto or Jupiter would affect anything but it probably does.

If it helps, I've made this incredibly stylish and oh-so-fabulous template in case anyone wants to sketch it in Paint or something. The black dot at the center is the celestial north pole.

Thank you for your time!


texts:
In a recent observation reported in the ScienceAlert article 'Boron has been detected on Mars for the first time" reporting that

  The Curiosity rover has found boron on the surface of Mars, indicating that, at some point, the Red Planet had long-term habitable groundwater.

Making the analogy that

  Boron here on Earth is associated with sites where there was once lots of water, but it's since evaporated away - like California's Death Valley.

However, they state that this may not necessarily be the case for Mars, so

What could be the possible origin of boron found in the Martian soil if not from ancient groundwater?


texts: Why is detecting brown dwarfs difficult?

texts: Why is detecting brown dwarf stars tedious and not always successful? They emit light in the infrared region, and given that we're surrounded by state-of-the-art technology in space today, why haven't the success rate grown? Why aren't all of the infrared-capable telescopes able to see them?


texts: How can I tell if my solar glasses are up to ISO standards?

texts:
Like many, we bought solar glasses to view the eclipse, and are now in a strange place where we can't tell if they're safe or not.  The big fuss seems to be about ISO 12312-2 compliance.  Apparently there are some glasses which are claiming to be certified and putting the logo in the glasses when they are, in fact, not up to spec.

Now the only "solution" that has been given is to buy from one of the "reputable" vendors documented by the AAS.  Many smaller vendors are crying foul, saying the big guys are just trying to push out competition.

So my question is this: if I had a pair of glasses, what would be the proper way of determining if they met ISO 12312-2 standards?  Is there supposed to be a way for a customer to trace their purchase back and verify that the company that made them at least has their paperwork in order? (A company that has their paperwork and is still counterfeiting anyway is a different challenge).

Obviously the astronomy community is interested in ensuring their products are safe.  How were they supposed to do that in the ideal circumstances?  If I can't confirm their certification as a small-time buyer, how would a large-scale buyer of, say, 100,000 glasses or viewers determine that the product they are buying is genuine before someone burns their eyeballs?  Surely when there are hundreds of thousands of dollars on the line, there would be a process to demonstrate compliance.

EDIT: I am specifically looking for how one is supposed to verify these glasses without relying on a whitelist provided by a single group such as the AAS (other than perhaps the ISO community itself).  Surely there is a way to verify claims of ISO compliance without relying on the good intentions of a professional society.  Otherwise, what is the point of having certifications in the first place?


texts: Did a galaxy get discovered more than 30 billion light-years away? How does one observe light older than the estimated age of the universe?

texts:
I'm more into questioning mainstream science and pseudo-science to remain open-minded.

Why is it that we are seeing light 30 billion light years away; when the estimated age of the universe is much younger?
How does space-stretching circumvent the light-speed limit?


texts: why has not Nasa gone back to our Moon since the first landing on the moon


texts: Why can I see sometimes a horizontal half moon instead of a vertical one?

texts:
Why do I sometimes see a horizontal half moon, instead of the more usual vertical one? I keep getting answers for crescent moons, but this is a half moon with its curve closer to the horizon, like a bowl of cereal.
I would like to know why it only appears sometimes that way.


texts:
A line from one of the answers on a different question got me thinking:

  The simplest way to see this is probably that a black hole has a much
  higher entropy than a star or even another type of stellar remnant of
  even vaguely similar mass and so there simply could not exist a
  spontaneous process by which a black hole develops back into a star.

Now, I agree a black hole turning into a star seems far-fetched since it's a one-way trip (like you can't recover a block of sugar from a glass of water to that exact form). But as far as I know entropy is the amount of disorder. A black hole is denser than a star. For a density that high, I assume a certain amount of order (inverse entropy?) is required. It's an enormous amount of mass in a small amount of space, keeping itself together. Sounds to me like a system, not like a random collection of mass.

How can the amount of order necessary for such dense objects as black holes be lower than that of the star they originate from?


texts: Does the CNO cycle play a role in stopping the collapse of solar mass stars after they have run out of Hydrogen?

texts:
Wikipedia says about Stellar evolution:

Eventually the star's core exhausts its supply of hydrogen and the
star begins to evolve off the main sequence. Without the outward
radiation pressure generated by the fusion of hydrogen to counteract
the force of gravity the core contracts until either electron
degeneracy pressure becomes sufficient to oppose gravity or the core
becomes hot enough (around 100 MK) for helium fusion to begin. Which
of these happens first depends upon the star's mass.

However, when I look at the article about Triple-alpha process, I find the diagram below. It shows that the CNO cycle is activated at lower temperatures than the Triple Alpha process. During their main sequence life, CNO cycle was much less dominant than PP chain reactions, so if the star had at least some amount of C and H (the ingredients for CNO cycle), I would expect that to turn on before triple alpha process.
Given that collapse causes temperatures to rise, does the CNO cycle turn on when the temperature of the star is higher than what it was during its main sequence lifetime, when the CNO cycle is more energy efficient than both PP chain and Triple Alpha process? This is roughly in the middle of the diagram below.
I can see a few reasons why it wouldn't turn on:

If the star runs out of H, it runs out of it so completely that even CNO cycle wouldn't ignite
Collapse happens so quickly that temperatures reach the regime where the Triple Alpha process is much more energy efficient than CNO Cycle, so that will be activated.

By energy efficient I mean producing more energy at that temperature.


texts: Why do small mirror imperfections matter with modern computers

texts: Modern telescopes go to great lengths to have perfectly shaped parabolic mirrors. My question is, why go to the trouble of having a perfect mirror? Why not take a mirror roughly the right shape, and then correct for the distortion using computers?


texts: How much more life could the Sun acquire via star lifting?

texts:
It has been proposed before that we could use this technique to remove hydrogen from the Sun to lower it's rate of fusion and extend it's life, so it doesn't fry our planet.

I am wondering how much more life could we possibly get out of the Sun doing this? I am thinking we will want to keep the Suns luminosity constant, perhaps lower than it is now even, it used to be only 70% but perhaps that is too low without us having to have too much carbon dioxide in the atmosphere for comfort (1000 ppm is the limit before we start getting affected iirc).

Anyway, in order to to that I believe the star lifting would have to be done regularly over the future lifespan of the sun so a very long term project. It has to be regularly tuned so to speak as the core depletes it's fuel supply and collapses a little, we will have to remove a little more material to restore balance to keep the same power output.

Does anyone know how to figure out how long we could keep doing this?

There will then of course, come a time when we can't remove more fuel from the sun because the core will have exhausted all accessible hydrogen by then and will have to resort to helium fusion.

Or can we keep the suns output constant even with helium fusion? Or do we by then have to consider radically different methods, like somehow removing the spent fuel from the core and replacing it with the hydrogen we removed the previous billion(s?) of years prior?


texts: Is it true that black holes emit only photons, electrons, and positrons in the form of Hawking Radiation?

texts: According to this comment, if the peak photon energy is 511 KeV, electrons and antielectrons begin to be produced along with photons. So when the peak energy is ~940 MeV, the rest mass of a proton, will they be produced as well, and what types of subatomic particles can be generated via Hawking Radiation?


texts:
I've read (at here, among other places) that during the Degenerate Era, star formation will end and the last stars will go out. But it was noted that there is still the possibility of star birth, coming from the collision of two stellar remnants, such as brown dwarfs (although white dwarfs also have the potential for such a collision). When two brown dwarfs collide, if they have enough combined mass, fusion can begin.

This is much different from how normal stars form today, and how they fuse hydrogen and other elements. So my question is this:

Will there be any major differences between the fusion process in the new "star" and a star with the same mass that formed via traditional star formation (i.e. collapse of a gas cloud)? Also, how will this affect the new "star's" life cycle?


texts: Which software is appropriate to integrate radial velocity with respect to time

texts:
I'm a high school student and currently I'm trying to use Baade-Wesselink method to independently derive period-luminosity relation of Cepheid variables. I noticed that to get the linear radius variation of a Cepheid, I need to integrate its radial velocity over its period.

The problem is, I don't know which program/software I should use to plot the best-fit line going through the points of the raw data of radial velocity over the phase. Even more, the thing is I need to integrate that best-fit line and plot the integrated curve as well.

Can anybody recommend good software that I can use?

(I've tried the integration with a program called Logger Pro but it was not successful. )


texts: Spinning black holes acting as a slingshot

texts: Could we use spinning black to act as a skyhook giving us enough energy boost to travel the galaxy?


texts: How to calculate the expected surface temperature of a planet

texts:
I'm writing a program to generate solar systems but I'm having trouble calculating the expected temperature of a planet. I have found a formula to calculate this, but I haven't been able to get a remotely correct answer out of it as it doesn't clearly state what units your supposed to use.

This formula I found:

$$4 \pi R ^ 2 ơ T ^ 4 = \frac{\pi  R ^ 2  L_{\odot}(1 - a)}{(4  \pi  d ^ 2)}$$

where $R$ is the planet's radius (not sure what units), $d$ is the distance from the Sun (it mentions AU), $a$ is the albedo, $L_{\odot}$ is the luminosity of the Sun (which I assume can be interchanged with the luminosity of any star), $T$ is the temperature of the planet (kelvin, this is what I'm trying to get), and $ơ$ is the Stefan-Boltzmann constant.

The site I found it on is notes for an astronomy college course. Here is the link:

http://www.astronomynotes.com/solarsys/s3c.htm#

Any help would be very much appreciated.


texts: What is the range of the zenith angle between velocity and position vectors?

texts:
I'm coding on a space simulator and I want to draw a predictable orbit for a satellite. I have read about orbits on http://www.braeunig.us/space/ and under the section "Launch of a Space Vehicle" they describe how I can obtain the orbital elements like eccentricity, true anomaly etc. given that you have the position vector, velocity vector and the zenith angle between them.

I have two questions that I need to understand:

What is the range of the zenith angle (0-180 degrees or 0-360 degrees etc.) ?
Can the angle tell which "half side" of the orbit the satellite is? I.e. if it's onwards to periapsis or apoapsis?


texts:
I was reading that:

  However, unlike Galactic sources such as W3(OH), the emission is unpolarized and the 1667 MHz line is stronger than the 1665 MHz line.

but how is this possible? Does not the 1665 MHz line and 1667 MHz line correspond to a particular transition and dipole moment? Won't emitted photons naturally have a polarization?

Furthermore, they state:

  The characteristics of the λ18 cm OH mega-maser emission differ from those of Galactic maser sources, such as the main line intensity ratio (T1667MHz/T1665 MHz > 1), large linewidth (>100 km s^−1), and unpolarized emission.

But why is unpolarized emission expected from extragalactic masers (arising near AGN) and not galactic ones (arising in circumstellar and interstellar environments)?


texts: What is the maximum Clear Sky Light Pollution rating to see the 2018 Perseid Meteor Shower?

texts:
I'm talking about the Light Pollution ratings on this page: http://www.cleardarksky.com/csk/faq/2.html

I live in an area which is a 7 on the Bortle Scale ("red" on clearsky.com), but within 2 hours of an area with a rating of 3. I am trying to determine how far I need to drive in order to get a proper viewing.

What is the maximum rating that will enable me to view the Perseid Meteor Shower tonight (2018)?


texts: Special Relativity as Applied to an Interstellar Starship?

texts:
I'm once again confused by the twin paradox. Let's say I am on an interstellar starship flying at 0.6c from a star 30 light years away from Earth to a star 50 light years away from Earth and 40 light years from each other. How would you estimate the difference between the time on the ship when you reach the destination versus the time a person on Earth thinks it should be? Now let's say I travel back to the original star (In both circumstances, I accelerate at 1g until reaching 0.6c.) I would assume that I could use the twin paradox formula to determine the difference in elapsed time on the ship from that measured by a clock left at the first star.

Now, let's say I'm traveling in the same manner (1g acceleration until reaching 0.6c and 1g deceleration for the same duration until arrival) beginning at Earth, traveling to 16 stars and returning to Earth. I am thinking that the time difference between the ship clock (date) and Earth time would be based on the distance and acceleration/ deceleration segments of each leg of the journey independent of the components of the travel vectors in the direction (toward or away from the Earth). Am I right??

Would Earth send messages to the ship based on conventional measurements (ie Physics 101)? I would want to determine the difference between the time on Earth when the message is sent and the ship time when the message is received, considering of course the time of transit of the message (at light speed).


texts: Where does the CO2 in Mars atmosphere come from?

texts:
As far as I know, Earth and Mars were relatively similar when the solar system formed. So most of the CO2 was used to form carbonic rocks (right?). I found that tectonic movements are the reason that the CO2 on Earth could be released again, which don't exist on Mars.

So how comes that the atmosphere has 95% CO2?


texts: Would Adaptive Optics be Useful in Radio Astronomy?

texts:
This Question and Answer got me thinking. If atmospheric seeing at visible wavelengths is the result of refractive index inhomogeneity, would it also be a similar problem for mm to cm wavelengths? From a quick search, the index of refraction of air at STP is about 1.0003 (visible) and 1.0002 (radio).
If it is not, is there a way to understand quantitatively why it is not a problem?

Sources: 1, 2


texts: Why Tidal Tails in Disk Galaxies, but not in Ellipticals?

texts:
Why do tidal tails result from only disk galaxies but not ellipticals.

I understand that tidal tails are results of gravitational interactions not strong enough to completely plunge out the material violently. How is that they form only in organized motions (disk) rather than random motion (ellipticals).

Any references to a detailed explanation is highly appreciated.


texts: What kind of things I could "see" with an amateur radio telescope?

texts:
There are apparently not many reasonably priced radio telescopes available for the amateur users. I only could find a SPIDER 230C 2.3 meter diameter compact radio telescope, which costs ~10 k€. Reading the page, I don't get a good overview what kind of things I could "see" with a radio telescope that has a 2.3 meter antenna.
Is the instrument already a (semi-)professional one?


texts:
Where could I find detailed data from stars? Like spectrum, x ray and so on ?

Because I would like to do some data analysis on it.


texts: Has someone ever captured footage of the collision of a meteoroid and a meteoroid?

texts: For a video game I'm looking for footage of a collision of meteoroids or maybe asteroids, not planets. Even though I have searched for a while I couldn't find anything. So has anyone ever captured footage?


texts: Ecliptic plane, its precise definition useful for calculations

texts:
My question is similar to the one posted by Abtract, whom however never got a useful answer. I am trying to calculate from scratch the exact times of sunrise and sunset anywhere on earth taking into account the 'equation of time' and other time dependent variables. This requires computing the line of sight to the rim of the sun at any universal time instant. To do this one needs an invariant as possible coordinate systems, preferably centered at the sun. I have tried using the ecliptic plane and its normal as basis, but with only moderate success (up to about a minutes difference from published values). Now many texts define the ecliptic plane as the one which contains the yearly path of the earth around the sun or, vice versa, the apparent path of the sun around the earth.
However, unless stated as an "average path" over a certain (long - say 100 years) time frame, then the normal of this plane must wobble monthly due to the pertubations caused by the moon (ignoring the other planets) and the intersection between the earth's equatorial plane and the ecliptic also varies. So is the ecliptic actually the path of the earth-moon barycenter (in which case it would be more invariant) or is there another more stringent definition I can use to get accurate timings? So exactly what is the best polar coordinate system and what are it's commonly named coordinates and parameters.


texts: How to choose portable, yet powerful telescope

texts:
Due to the nature of my job, I have to spend log periods of time (4 - 6 months each), at different countries across Europe, Middle East and Far East. I have a 12 Inch Dobsonian at home, but I rarely spend 1 month a year, at home. Travelling with the Dob is out of question due to its weight and I don't always have access to low lying windows or a rooftop when abroad, making a dob pretty useless. I really want to have at one, at least 8", portable option while I'm travelling. Celestron NexStar 8 SE is looking good to me at the moment. It seems to have half the weight of an 8" scope with equatorial mount and from what I've heard, it can be used table-top, without tripod. I can also carry the OTA in hand luggage, to avoid damage to optical components. Can someone confirm if

(i) Does 8SE work on tabletop without tripod?
(ii) How durable is the mount? Can it survive transportation in checked baggage, if placed in its original packaging?
(iii) Any other suggestions for a scope that is at or above 8" and can be carried around easily when travelling internationally?


texts: What would happen on earth if there was a surge in gravitational waves

texts:
There are claims that when massive objects merge some of their mass is converted to gravitational energy. Such as the following links:
https://astronomy.stackexchange.com/a/38727/2305
https://youtu.be/FtYwsw3hefc
What kind of macro effects, if any, could be perceived by a person with their senses without instruments if sufficiently large gravitational waves happened to hit earth, in the case of a ripple and in the case of a long term constant change (if that's even a thing, otherwise please assume a very very low frequency, which would mean a gentle slope).
What would be an example of the distances and masses involved for us to feel such effect?


texts: Impact of Atmospheric Water vapour on Optical Observations

texts:
I would like to ask; how does Water Vapour in the atmosphere interfere with astronomical observations? Is it in the same manner as Carbon dioxide?

What brought this to mind is the following: A member of Monash University staff claimed recently in a newspaper in Australia that both CO2 and H2O absorbed heat and hence their presence needed to be reduced for astronimical observations. Hence the siting of optical observatories on mountain tops where the absolute amount of water vapour in the atmosphere is reduced by the relatively low temperatures there.


texts: Why is this HST WFPC2 image so much fuzzier than the WFC3 image of the same field?

texts:
I was looking at the comparison of 2007 and 2015 HST images of the "disappearing" star N6946-BH1 in Phys.org's article Collapsing star gives birth to a black hole. The 2007 images is from the WFPC2 while the one from 2015 is from WFC3.
I wondered why the WFPC2 image is substantially fuzzier than that from WFC3. I looked at the WFPC2 manual for cycle 17 and found it has the "L-shaped" detector. Three quadrants have CCD elements with pixel spacing of 0.1 arcsec, the fourth "planetary" array has a pixel spacing of 0.046. However it seems more complicated because - if I understand correctly - there are different focal ratios used for the two.

The WFPC2 field-of-view is divided into four cameras by a four-faceted
pyramid mirror near the HST focal plane. Each of the four cameras
contains an 800x800 pixel Loral CCD detector. Three cameras operate at
an image scale of 0.1″ per pixel (F/12.9) and comprise the Wide Field
Camera (WFC) with an “L” shaped field-of-view. The fourth camera
operates at 0.046″ per pixel (F/28.3) and is referred to as the Planetary
Camera (PC).

However, while the WFC3 manual for cycle 23 gives the pixel spacing as

Table 5.1: WFC3 Detector Characteristics
0.04″ per pixel

I didn't find a focal ratio.
I'd like to understand why the 2007 image is so "gaussian fuzzy". If it were only because of larger pixel spacing it would look more blockish or pixellated. Does this mean it was using a different optical path with lower optical resolution? Is the 2007 image from the WFC part at F/12.9? Does the WFC3 use F/28.3?
I also don't understand the optical reason why different paths would have different optical resolutions. This would not come from different Airy disks from simple diffraction - the angular size would be unchanged.

above: cropped sections from the same image shown below.

above: From here.


texts: Is there a possibility of finding a system where a star is orbiting a planet?

texts:
Is it possible for a star to orbit a planet IF the planet is bigger than the said star?
Or is it possible even if the star is bigger than the planet and still the star is orbiting the planet?
I have done some research with no results.


texts: What's the likelihood of the existence of unknown elements in the Solar System?

texts: What's the chance that there might be undiscovered chemical elements in the Solar System - either on planets or around the Sun or on asteroids of the Oort-cloud?


texts: Can a solar eclipse be directly overhead?

texts: I have trouble picturing eclipses and whether the moon can be directly overhead.


texts: What is it that I have been seeing in the sky?

texts:
So I'm interested in space scientifically. I do have a firm belief that there could be intelligent civilizations in our universe. Just wanted to get that out of the way.

I've been spending the evenings in the garden due the weather. Past two days I've seen the same similar thing and I'm not sure what it is. I have observed many 'shooting stars', planets, satellites, and aircraft. It's just one solid bright white light. I actually checked it with binoculars I had in the kitchen an yeah all I can see is bright white light. It is much brighter and larger to my eye than satellites appear. It is about 22:00 when I see these it is pretty dark but I wouldn't say peak darkness. It moves steadily faster than large aircraft like commercial flights that I can see. It moves in a straight line I think and is fully visible but just as that bright light.

I could possibly go to lengths and get images of it if anyone wants I took video on both occasions. They are pretty clear and I tried adjusting stuff on the camera to improve the quality while filming.

Anyway, would be interested in what it could be obviously. Responses will be appreciated.

EDITS:

Okay, so the town I'm in is called Colchester, United Kingdom.
I was facing West, it came from above the cloud, was to the right side of Bootes. Carried on in a straight line across the whole sky travelling East until it was no longer visible. The light remained bright the entire time.

Oh, and 22:00, GMT.

I am uploading the videos and images, I will link to it.


texts: Where can I find data on Mercury's orbit to fit a model to?

texts: I have a project in which a Monte-Carlo method is used to find the optimal parameters for some model that is fit to a data set. I want to fit a model for the orbit of Mercury using this methods, but I can't find any data to fit it to. I have found a lot of papers on the orbit of Mercury that mention observational data, but no actual observational data. Where could I find data indicating the position of Mercury around the sun over a time period, or even a relatively accurate mathematical model from which I can generate my own data by defining it as a function that is stepped through some time interval?


texts: Why does the outside edge of a Kerr black hole ergosphere rotate at the same speed as the inner edge?

texts:
Reading through the Wikipedia on a Kerr black hole ergosphere, and this nice question on the maximal spin rate of a black hole leads me to ask: why does the outer edge of the ergosphere rotate at the same rate as the inner edge?

Imagine I'm flying into a spinning black hole and haven't yet crossed into the ergosphere. Space-time is rotating more and more as I approach the black hole. I'm rotating at 90% the speed of light, then 91% etc. until I cross into the outer edge of the ergosphere and am rotating at 100% the speed of light. My question is, how does this continue at exactly 100% for a while before I enter the Schwartzschild radius and fall in for good?

My intuition would be that rotation would be a continuous smooth function, rather than a 'step' function where there's a zone where I'm getting closer to the black hole, but not rotating any faster. Where is the extra energy going?


texts:
NASA has a page dedicated to lunar eclipses and provides charts. How are these charts to be read? What do P1, P4, U1,... mean? What about the world map below?

Click for full size view


texts: Do we know what happens at the boundary of the universe, what are some theories about it? Does space time differ at the boundary? Do things become larger at the boundary? Do the trajectories curve and force you to remain at the edge instead of crossing it? What are some theories about the edge of the universe?


texts: Is there any star recognition framework/library available?

texts:
Being a daytime software engineer and a sparetime photographer, I was looking for a nice DIY project for the next weeks. I finally decided to build a star tracker with a camera mount, so that I can take deep sky pictures with my DSLR.

While planning and layouting the components for this, I had to decide whether I want to manually adjust the rotation axis of the device so that it aligns with the rotation axis of the earth - or go the nerd way and let the system auto-arrange itself by attaching a suitable fixed USB camera, evaluate the video/images and write some software to move the device with stepper motors and screw drives until the correct orientation is reached.

As I do not want to reinvent the wheel, is there any star recognition framework that you've heard of? Preferrably open source, the language is of marginal importance as long as I can get the relevant data in and out. I'm thinking of something that I can feed with an image file and get back an array with the recognized objects with their x/y locations within the image. I did some research on that but could not find anything promising. What I need is an automatic identification of the brightest stars, so that I can check the current orientation against a reference image and get some vectors for the stepper motors. I hope I got the idea across.


texts:
I heard that Jupiter is made out of gas. But in school I learned that Jupiter has gravity which is 2.5 times that of Earth (Gravity that can tear apart a comet) and gravity is proportional to mass.

So if Jupiter is made out of gas alone, how does it have such high gravity and maintain so many moons around it?


texts: Brightest star in the Eastern sky 2017 in am

texts: What is the name of brightest star in the East 2am.  What does a satellite look like?


texts: Okay, so in this ArXiv report from 2014, scientists discovered the coolest white dwarf, with a temperature of below 3000 K. However, as this report was from over 6 years ago, I think this information may be outdated. So, as of right now, what is the coolest white dwarf, and where can I find more information about this?


texts: Get the expression of probed volume between 2 redshifts

texts:
1) I can't manage to find/justify the relation (1) below, from the common relation (2) of a volume.
2) It seems the variable r is actually the comoving distance and not comoving coordinates (with scale factor R(t) between both).
The comoving volume of a region covering a solid angle $\Omega$ between two redshifts $z_{\mathrm{i}}$ and $z_{\mathrm{f}},$is given by
$$
V\left(z_{\mathrm{i}}, z_{\mathrm{f}}\right)=\Omega \int_{z_{\mathrm{i}}}^{z_{\mathrm{f}}} \frac{r^{2}(z)}{\sqrt{1-\kappa r^{2}(z)}} \frac{c \mathrm{d} z}{H(z)}\quad\quad(1)
$$
for a spatially flat universe $\kappa=0)$ the latter becomes
$$V\left(z_{\mathrm{i}}, z_{\mathrm{f}}\right)=\Omega \int_{r\left(z_{\mathrm{i}}\right)}^{r\left(z_{\mathrm{f}}\right)} r^{2} \mathrm{d} r=\frac{\Omega}{3}\left[r^{3}\left(z_{\mathrm{f}}\right)-r^{3}\left(z_{\mathrm{i}}\right)\right]
\quad\quad(2)$$
I would like to demonstrate it from the comoving distance with :
$$D_{\mathrm{A}}(z)=\left\{\begin{array}{ll}
{(1+z)^{-1} \frac{c}{H_{0}} \frac{1}{\sqrt{\left|\Omega_{\mathrm{K}, 0}\right|}} \sin \left[\sqrt{\left|\Omega_{\mathrm{K}, 0}\right|} \frac{H_{0}}{c} r(z)\right],} & {\text { if } \Omega_{\mathrm{K}, 0}<0} \\
{(1+z)^{-1} r(z),} & {\text { if } \Omega_{\mathrm{K}, 0}=0} \\
{(1+z)^{-1} \frac{c}{H_{0}} \frac{1}{\sqrt{\Omega_{\mathrm{K}, 0}}} \sinh \left[\sqrt{\Omega_{\mathrm{K}, 0}} \frac{H_{0}}{c} r(z)\right]} & {\text { if } \Omega_{\mathrm{K}, 0}>0}
\end{array}\right.
$$
UPDATE 1: It's been a long time that I posted this question.
I recently took over this isssue and I have done little progress, at least I think.
The FLRW metric can be expressed under following (0,2) tensor form :
$\left[\begin{array}{cccc}1 & 0 & 0 & 0 \\ 0 & -\frac{R^{2}(t)}{1-k r^{2}} & 0 & 0 \\ 0 & 0 & -R^{2}(t) r^{2}  & 0 \\ 0 & 0 & 0 & -R^{2}(t) r^{2} \sin ^{2} \theta\end{array}\right]$
If I consider only slice times constant, my goal is to compute the volume probeb by a satellite between 2 redshifts.

We can easily find that :

$$\int_{0}^{z_0}\frac{cdz}{H(z)} = \int_{0}^{t_0}\frac{cdt}{R(t)}$$

Then, If I consider a volume with $\text{d}r$, $\text{d}\theta$ and $\text{d}\phi$ coordinates, I have the following expression for determinant :

$$g=\text{det}[g_{ij}] = -\dfrac{R(t)^6}{1-kr^2}\,r^4\,\text{sin}^2\theta$$
Which means that I have :
$$\text{d}V=\sqrt{-g}\text{d}^3x = \dfrac{R(t)^3}{\sqrt{1-kr^2}}\,r^2\,\text{d}r\text{sin}\theta\,\text{d}\theta\, \text{d}\phi$$
$$V=\int\text{d}V= \int \dfrac{R(t)^3}{\sqrt{1-kr^2}}\,r^2\,\text{d}r\text{sin}\theta\,\text{d}\theta\, \text{d}\phi$$
$$V = \int \text{d}\Omega \int \dfrac{R(t)^3}{\sqrt{1-kr^2}}\,r^2\,\text{d}r$$
$$\rightarrow\quad V = \Omega \int \dfrac{R(t)^3}{\sqrt{1-kr^2}}\,r^2\,\text{d}r$$
with $\Omega$ the solid angle considered.
But as you can see, I am far away from the expression $(1)$ that I would like to find, i.e :
$$V\left(z_{\mathrm{i}}, z_{\mathrm{f}}\right)=\Omega \int_{z_{\mathrm{i}}}^{z_{\mathrm{f}}} \frac{r^{2}(z)}{\sqrt{1-\kappa r^{2}(z)}} \frac{c \mathrm{d} z}{H(z)}\quad\quad(1)$$
EDIT : Maybe I have found a partial explanation to my issue to determine the expresion of this volume between 2 redshifts. Here below a formula :

The main expression to keep in mind is :
$$d V_{C}=D_{H} \frac{(1+z)^{2} D_{A}^{2}}{E(z)} d \Omega d z\quad(3)$$

Could anyone explain me please the different justifications to introduce all the factors implied in this expression ?

I have not yet with this expression the same expression (1) at the beginning of my post, so could anyone manage to find (1) from (3) ?

Any help would be fine, I am stucked for the moment.


texts: What is the exact time Earth takes to revolve around its axis?

texts:
I read that our day is not exactly 24 hours long. If it is not then why weather seasons come at the same time (Summer, Winter, Autumn Spring) ? how it is equalized ?

Update: Sorry I could not make it clear. My meaning was if 3.8 seconds are shorter of everyday the how is the calculation made that summer starts in May and winter start december everytime why it does not move further since. How time is neutralized ? I hope you guys understood this time.


texts: How do you call it when two celestial bodies come as close to each other as they will in their current orbits?

texts:
On Earth, we say Mars is in opposition when it is 180º from the Sun, which also marks the times when Mars and Earth come closest to each other.

Is there a specific name for such proximity events? For example, Saturn and Jupiter come to this maximal proximity every 20 years or so. Is there a name for the event itself, the "approaching" (previous) phase and the "departing" (next) phase?


texts: Why is the Sun's Luminosity not equal to the Zero Point Luminosity?

texts:
I was doing some research and found that the zero-point luminosity was $L_\odot = 3.75\times 10^{28} $, while the sun's reported luminosity was $L_{Sun} = 3.828\times 10^{26}$, according to Wikipedia. Why isn't it the same value? Doesn't the symbol $\odot$ represent the value of our sun?

Furthermore, when Googling the Luminosity of the sun, it gives $1L_\odot$, which suggests that $L_{Sun} = L_\odot$ when it is evidently not. What's going on here? I'm very confused.

For background, I have little to no background knowledge of astrophysics, and I'm trying to learn about it.

Thanks.


texts: What is the role of the mesh on which dipole elements of the MWA antennas are placed?

texts:
I was reading about the radio telescope - Murchison Widefield Array (MWA) situated in Western Australia. Antennas of this telescope are quite unique and different from the usual dish radio telescope. In MWA a four by four regular grid of dual-polarisation dipole elements are arranged on a 4m x 4m steel mesh.

So, I was wondering why these dipole elements would have been mounted over the mesh? On searching over internet I found out that this mesh is called a counterpoise (though I am not sure if I am correct). I read more about it and I partially understand that the mesh and ground (Earth) act as two plates of a capacitor but I didn't get why it was done and what was the use of it? In this image mesh can be seen on which dipole are mounted - Image courtesy: SKA

 Source  click for full size


texts: How many horizontal branch stars are within 2 kpc of the sun?

texts: For my research purposes, I am wondering how many horizontal branch (HB) stars are within 2 kpc of the sun?  I tried to find some with the Gaia DR2 data with a magnitude cut that leaves HB stars out if they're beyond ~2 kpc, and there were surprisingly few unambiguous HB stars, so I'm just wondering how many there are and/or how to go about estimating that.


texts: Does the mass of a collapsing star change?

texts: Does a star, when collapsing into a neutron star, or even a black hole, pick up any significant relativistic mass due to its rapid collapse rate?


texts: Where can I find the observed frequency of triple star systems where the outer star is in a retrograde orbit?


texts: Is there a term to describe all stars that are not neutron stars or white dwarfs?

texts: I am looking for a official term that describes all stars that are not neutron stars or white dwarfs.  "Still burning stuff stars" or "ordinary stars" are all I have been able to come up with.  Is there an official term for stars that are still evolving and haven't "died" yet?


texts: How much light would be reflected from Jupiter to Europa (in Europas night)?

texts: Suppose Europa is found habitable and humans start living on it. Europa is close to Jupiter and during the night Jupiter would be reflecting light to Europa, just like our full moon reflects light to earth. Our moon is small compared to Jupiter. Would humans still need artificial light during Europas nights?


texts:
Given that moons commonly orbit planets, why do we never encounter a comet orbiting a planet?

What would happen (exactly, in detail) if a large one did settle into a stable orbit around the Earth?

Sorry... just making the question a bit clearer!  The accepted answer will describe where all the water ends up.


texts: Why can't we travel faster than light? And is the theory of relativity absolutely flawless?


texts: How much does the stellar radius change if seen through different filters?

texts: I know that the radius of a star should be different depending on the filter being used to do the observation. That is, the bluer the filter, the smaller the measured stellar radius. I know this effect must be pretty small, however, I would like to get more familiar with it. Can anyone point me to some literature or share their own experiences? Would it be a significant difference in Solar radius if we observe the Sun in x-ray vs radio bands?


texts: How does a gravity slingshot actually work?

texts:
From what I know of elliptical orbits, an object speeds up near the periapsis and slows down at the apoapsis, much like we learned in high school physics how a sphere would roll down and back up a valley in a frictionless vacuum: The height is inversely proportional to the speed.

The "gravity slingshot" maneuver we've seen in sci-fi and even used by our own spacecraft relies on the physics of hyperbolic orbits, where an object both enters and exits the orbit before making a single lap around the planet/moon/etc. Since gravity pushes the craft toward that body both while they're heading towards and away from it, shouldn't the craft's velocity be the same at (for example) 1 megameter before the periapsis as 1 megameter after? If so, then the gravity slingshot maneuver should only have the end purpose of redirecting the craft's trajectory, not of increasing its speed, as the name implies.

My understanding in a simple diagram:


texts: If Jupiter is made of gas, could we fly or drive through it or would its center be too dense?


texts:
Cosmic horizon in the de Sitter space is a sphere, centered at the observer with finite radius where the red shift due to cosmic expansion becomes infinite.

Given that no information can be transmitted from behind the cosmic horizon, I wonder whether any matter can ever pass behind the event horizon, because this will give rise to an information paradox, similar to the black hole information paradox.

The matter approaching the cosmic horizon seems to experience time dilation, so will that time dilation prevent it from crossing the horizon?

To work around the paradox, one have to postulate that the information about outgoing matter should be encoded in the de Sitter radiation.


texts: Why is Enceladus's albedo greater than 1?

texts: Enceladus, one of Saturn's moons, has a geometric albedo of 1.38 and a bond albedo of 0.81. How can the geometric albedo of Enceladus be greater than one? What are the similarities or differences between a geometric albedo and a bond albedo?


texts: What Constellation Is This? (Photo Included)

texts: Observed on 9th of Jan 2021 in Newcastle Upon Tyne (UK) looking WSW at a little after Midnight.


texts:
I am reading about massive stars and their final consequences. what I understood is massive stars after throwing out their outer envelop with masses > 1.4 solar Mass turn into neutron stars. But I also know that Massive stars can collapse to form a type II supernova. I need to understand how we determine which stars go through which final fate. And what initial mass they have to start their life with for both fates.

I ma be completely wrong with my understandings. Your clarifications will help a lot.


texts: What if the Sun turned into a red giant tomorrow?

texts:
Let's say that, hypothetically, our sun uses up all of it's hydrogen tonight, and tomorrow morning is the first day of its red giant phase. How long would it take for the star to turn red? How long before we start noticing adverse climate conditions and runaway heating of the Earth?

For a second question, let's say that using magical future technology, we are able to widen the Earth's orbit bit by bit, to keep it inside the goldilocks zone. How would the sky look on this new red-giant-orbiting Earth. How would plants begin to evolve and change color in order to adapt?


texts: How much of the ambient radiation on Mars is arriving line of sight from the Sun?

texts: I read a lot about the amount of radiation bodies like the Moon and Mars receive, but it's hard to find information about where the radiation is coming from. It seems to me that it's cosmic radiation plus whatever is being radiating from the Sun. Would it be useful to build a habitat somewhere that's in perpetual shadow (e.g. deep crater), or to seek shelter in the shadow of a crater wall or mountain during a solar storm. Mostly I'm thinking about a habitat design for Mars, where the habitat will always be out of view of the sun. Sunlight could light the habitat via something like huge Mylar mirrors, that would not reflect much harmful radiation. But I'd like to know if the applies to the Moon.


texts:
I recently inherited some Fortran 77 code that calls the IMFORT interface to write data out to an imh and pix files. I could not get this to compile on a modern compiler and it is not supported. So I decided to set off on writing a Fits writer in Fortran. I was able to make one but is ugly and does not properly handle the end card. I tried googling for a Fortran Fits writer but came up empty. Is there a library for astronomical Frotran codes that contains Fits writer exist out there.

The code may be better suited for stack overflow, but this is specialized to astronomy since no other science seems to use fits, so I figured I would be better off starting here.


texts: I had heard that Mars once had a planetary magnetic field, but that it is now gone? What happened to it?


texts: How to disentangle a very distant star's relative velocity vs. redshift distance

texts:
We measure a star's relative velocity towards or away from us via its Doppler-shifted spectrum. This is also how we measure the distance of very distant stars: measuring the shifts in the spectrum tells us its relative velocity, which we interpret as a distance via Hubble's Law.

Suppose we knew the star's relative velocity and distance exactly. It seems to me that if the star were moving away from us slightly less than would be expected from expansion alone, it would be because it was moving towards us more than the matter surrounding it: it would have some additional velocity component towards us. If we were to try to infer that star's distance from expansion alone, we would measure its relative velocity via the Doppler shift and would conclude that it was closer than it actually is. Conversely, if it were moving away from us faster than it should based on expansion alone, we would estimate its distance further than it actually is.

Is there any way to disentangle this degeneracy?


texts: Why is it impossible for a Quasi Star to exist these days?

texts:
I've heard about quasi stars a while ago, but I've just recently decided to go more in-depth about the characteristics and facts about them. I learned that they gain their energy through radiation when matter falls into the black hole in the core. I may be wrong about this part, though. I also learned that they can't exist these days because the universe's gases are polluted with metals. Why does that stop quasi stars from existing?
Edit request from comment: A quasi star is a hypothetical star that existed in the early ages of the universe. It got its energy from a black hole at its core, unlike normal stars that get energy through nuclear fusion.
https://medium.com/predict/quasi-stars-black-holes-at-the-core-of-the-universes-largest-stars-38bfbc4e1b95
If you would like more specifications, please tell me


texts:
There are a lot of photos of diffrent astronomial objects such as galaxies, nebulae and stars of which some are very far away.

However there are no real photos of exoplanets (by “real photos” I mean pictures that actually show something more than a blob of light). Why is that?

Distances of galaxies are by many orders of magnitude larger than distances to exoplanets and yet we have a lot of very detailed (and beautiful) pictures of them. Meanwhile, everything we got on exoplanets is limited to a few pictures with blurred blobs of light.


texts: Can I use the parallax coefficients for observatories as a proxy for latitude using arctan2(sin, cos)?

texts:
In How are the three character IAU Minor Planet Center's Observatory Codes assigned? Why are some letters so popular? I looked at the distribution of the first alphanumeric character versus the value of the second two (36 x 100).

The table lists longitudes but no latitudes for these observatories, but using the cos and sin parallax coefficients in $\arctan2(\sin, \cos)$ and plotting as a function of tabular longitude produces a map that looks like it could be the distribution of telescopes on Earth.

Question: What exactly are the cos and sin parallax coefficients, and how closely can $\arctan2(\sin, \cos)$ be expected to serve as a proxy for latitude? Within a degree or so?

import numpy as np
import matplotlib.pyplot as plt
# blob is the unformatted lines from https://www.minorplanetcenter.net/iau/lists/ObsCodes.html
threes = [line[:3] for line in blob.splitlines()]
key = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'
a = [[x] for x in key]
for t in threes:
    a[key.find(t[0])].append(t)
b = np.zeros((36, 100))
for i, thing in enumerate(a):
    c = [int(x[1:]) for x in thing[1:]]
    for d in c:
        b[i, d] = 1
plt.imshow(-b, interpolation='nearest', cmap='gray')
plt.gca().set_aspect(2)
plt.show()
pairs = []
todegs = 180/np.pi
for line in blob.splitlines():
    try:
        cos, sin = [float(x) for x in (line[13:21], line[21:30])]
        lat = todegs * np.arctan2(sin, cos)
        lon = float(line[4:13])
        pairs.append([lon, lat])
    except:
        pass
print(len(pairs), len(blob.splitlines()), float(len(pairs))/len(blob.splitlines()) )
lon, lat = np.array(list(zip(*pairs)))
lon = np.mod(lon+180, 360) - 180
plt.figure()
plt.plot(lon, lat, 'ok', markersize=1)
plt.ylim(-90, 90)
plt.xlim(-180, 180)
plt.gca().set_aspect('equal')
plt.show()


texts: Difference between quasar and Active Galactic Nuclei?

texts: Are there any differences between quasars and Active Galactic Nuclei (AGN)?  As I understand it, they are both caused by extremely luminous electromagnetic emission from material accreting onto a super-massive black hole.  If they have the same source, is there still some fundamental difference between them (and if so what is it) or are they two names for the same thing?


texts: Would a satellite that has a retrograde orbit and a shorter orbtial period than its planet's rotational period be tidally accelerated or decelerated?

texts:
There are 4 configurations to consider. A satellite orbiting a planet that:

Has a prograde orbit and larger orbital period than its planet's rotational period (example: The Moon)

Has a prograde orbit and smaller orbital period than its planet's rotational period (example: Phobos)

Has a retrograde orbit and larger orbital period than its planet's rotational period (example: Triton)

Has a retrograde orbit and smaller orbital period than its planet's rotational period (example: N/A)

For the 1st example, the planet's rotation will slow down and the satellite will receed into a higher orbit. This will keep happening until both the planet and satellite are tidally locked to each other. (Tidal acceleration)
For the 2nd example, the planet's rotation will speed up and the satellite will spiral into the planet. (Tidal deceleration)
For the 3rd example, the satellite will spiral into the planet but the planet's rotation will also slow down. Angular momentum is conserved because the orbit is retrograde. (Tidal deceleration)
What would happen in the 4th example? Is it a "negative times negative gives a positive" situation and we refer to the 1st example?


texts: Which quantum reality will be seen in a far away galaxy

texts:
I recently watched Brian Greene documentaries on parallel universes and am trying to wrap my brain around multiple quantum universes that form at every event and Einstein's relativity that in space-time, past present and future coexist so we can observe the future of a far away galaxy from Earth by moving.
https://www.youtube.com/watch?v=MO_Q_f1WgQI - Around 7th minute - explains how an alien moving towards earth has a now slice cutting in Earth's future

My question is which quantum universe will be seen because future events in that galaxy would create multiple such universes.

My question is regarding many worlds interpretation of quantum mechanics according to which multiple alternate futures are possible https://en.m.wikipedia.org/wiki/Many-worlds_interpretation

The question has been answered by James K below.


texts: How to watch at best this week's planet alignement?

texts:
I read somewhere that counting from today, five planets will be aligned during almost two weeks IIRC. The planets are : Mars, Jupiter, Saturn, Mercury and... us. I know Jupiter and Mars can be seen with the naked eye, but I would really like to catch a glimpse of Saturn.

Assuming one does not own a telescope, but small binoculars, how can one maximize his chances of seeing at least Mars, Jupiter and Saturn ?


texts: Would a naked eye supernova become a disc in a matter of days?

texts:
Stars are points of light even in telescopes- if they go supernova, their diameter does expand, but given the distance of even nearby stars, they would still be points of light, I suppose?
I am asking because this video suggests that a very large disc would be seen in the sky after a few days if Betelgeuse were to go supernova. How long would it take for us to see a disc instead of a bright spot?


texts: how to plot sky map like www.nakedeyeplanet.com with dates tags with planetary motion path

texts:
I am curious how to plot planetary motion star chart with date tags like on www.nakedeyeplanets.com by Martin J. Powell.

Is there any way to generate images similar like that. What software should be used MATLAB or what.

Also should have following features:
 - constellation boundaries
 - non-overlapping naming of star,constellation
 - different color for planet path for each year

Example, click for full size:


texts: How much ultraviolet light comes from the moon at night and from the Sun during the day? I am interested in using a UV camera on a robot to see the UV from the sun/moon to do robot localization.


texts: How do we know that our galaxy is a spiral galaxy?

texts:
I know that our galaxy is spiral in shape, but I'm wondering how the scientists found out that our galaxy has a spiral shape.

I don't think we can see the entire galaxy from telescopes on Earth, right?

I think it makes sense that they say that Andromeda has a spiral shape because we can see the whole galaxy, but how do scientist know about our galaxy?


texts:
Where can I find a large list of online star cameras with wide fields of view?

http://www.itelescope.net/all-sky-cameras is one example, but the images are fairly low resolution and there aren't many of them.

Is there a better site or sites to see wide-field star images?

I realize there are several online telescopes, but they all have a small limited field of view (which makes sense), so you can only see small portions of the sky at once, not the entire sky as above.


texts: I'm writing a report about the solar eclipse happening over north America and was wondering why looking at a solar eclipse is worse than looking at the sun regularly. In addition what alternate materials can be used to make eclipse glasses that are safe to look through.


texts: Confused about the significance of sidereal time

texts:
My understanding of sidereal time is that the position of a star will be in the same place 4 minutes earlier the next day. So a 20 days later the star will be 1hr 20 mins earlier. So if the star is in the same place it must be a multiple of 4 minutes earlier (and the seconds that I have ignored). So it is not possible for the star to be in the same place and not be that time multiple apart.
Its just that I have some data points where the locations are identical and they are all multiples of 4 mins apart but I am worried that that is not significant because it is inevitable. Is that correct?


texts: "How the Universe Works": is it scientifically accurate?

texts:
The documentary/mini-series How the Universe Works is based on scientific evidence and according to what is known nowadays, or sheer bunk?

Throughout the series, are there any particular topics which are still disputed among the scientific community and that were portrayed in a different light than they should've had been?

I've only watched a few episodes but they always talk in a way that gives you the feeling everything is well-established and accepted but that might not be the case. I figured that an astronomer that watched the series would have remember if this happened or not.

I understand the subjects were researched and there is footage of known scientists in the episodes, but that doesn't mean everyone agrees with them. (By everyone I mean the most part of scientists working on those subjects, and not literally every single one of them.)

To give you an example, on the second episode they state that there are massive black holes at the center of galaxies. From my perspective (i.e. of someone which is not an astronomer) if they say so it's because it's probably true. However, in the real world maybe this is a highly disputed theory and a lot of astronomers disagree with this stance, and they're note telling the audience about it. Why would they do that? I don't know. I don't know if they did, I'm just asking if someone knows whether it happened, or if anyone saw something in the series with which they disagree.


texts:
My textbook (and lots of other sources) shows the earth's longitude of perihelion at the epoch around 103 degrees. I understand that this means (in the neighborhood of J2000) the earth will reach perihelion about 103 degrees after the vernal equinox, however, this would be very close to aphelion (July 6ish) not perihelion (January 3ish).

Put another way: If the vernal equinox is March 20th & perihelion is January 3rd, shouldn't the longitude of perihelion be about -76 degrees (or 284 degrees)?

Help! What am I missing?

ANSWER--Credit to @Mike_G below, restated here because the final answer is in the comments: "At the March equinox, the alignment is Earth - Sun - first point of Aries." Beginner mistake: I was trying to find the earth's position in time but I thought the March equinox was when the earth was between the Sun & first point of Aries, which is the wrong arrangement.


texts: Why is the Color scheme of natural satellites in our solar system based of light shades of grey?

texts: What is the explanation of the color similarity that the majority of natural satellites obtain?


texts: Where did this famous Planetary Precession Formula come from?

texts:
The following equation (which I shall term the Planetary Precession Formula, PPF for short) famously appeared in a 1915  publication by Einstein where he indicated how it could be derived from his General Theory of Relativity (GTR).

$$\epsilon = \frac{24 \, \pi^3a^2}{c^2 T^2(1-e^2)}$$

where $\epsilon$ is the (anomalous, non-Newtonian) angular precession per orbit, $a$ is the orbit semi-major axis, $c$ is the speed of light, $T$ is the orbital period, $e$ is the orbit ellipticity.

The PPF formula accurately predicts the (anomalous, non-Newtonian) precession of Mercury and other Solar planets.

The formula was known in scientific circles well before 1915.  For example Gerber (1898) derived it from his own (widely-derided) model of gravity.  In the internet article Gerber's Gravity it is written that

  It became a fairly popular activity in the 1890s for physicists to propose various gravitational potentials based on finite propagation speed in order to account for some or all of Mercury's orbital precession.  Oppenheim published a review of these proposals in 1895.  The typical result of such proposals is a predicted non-Newtonian advance of orbital perihelia per revolution of... >

$$k\,\frac {\pi\,m}{L \,c^2} =  k \frac{4 \, \pi^3a^2}{c^2 T^2(1-e^2)}.$$

where $L = a(1 - e^2)$  is the semi-latus rectum of an ellipse, $m$ is a function of the angular speed $\omega$ of an orbiting planet: $m = a^3 \omega^2$ with $\omega = 2\pi/T$ and $k$ is a constant which might be derived from theory.

Clearly with $k = 6$ we get the PPF formula given above.

I wish to know where the $k\pi m/Lc^2$ expression comes from. From the article it would appear to come from the 28-page review paper by Oppenheim, 1895 which is scanned here. I have been through the scans of this paper but without finding that equation explicitly (the paper is in German which I know very poorly, Google Translate helps a bit but leaves a lot of ambiguity). It might be that the anonymous author of the article extracted the expression from a review of Oppenheim's paper or even the original (French & German) papers themselves, but he is not contactable.  Maybe someone here is familiar with this era of astrophysical history and can point me in the right direction?


texts: How can astrophysicists discriminate between pp-chain solar neutrinos and CNO-cycle ones?

texts:
Astrophysicists at the Borexino experiment in Italy have recently claimed that they have detected CNO-cycle neutrinos coming from the Sun.
It was the Cover story for the November 26 issue of Nature.
I read the whole Paper, and accompanying editorial, yet unless I missed something, they do not say what makes a CNO-neutrino different from an ordinary pp-neutrino.
They go to great lengths to describe how they excluded all types of background noise, but did not say whether CNO-neutrinos are higher or lower in energy than 'proton-proton chain' neutrinos....
Presumably, CNO neutrinos are higher in energy than pp ones?  How many eV or keV?


texts: How can I use my current camera whose zoom lens can't be removed behind an f/6 refractor for as an astrophotography?

texts:
I am looking at an Astrotech AT60ED (60 mm f/6 with a field flattener) and a Digi-Kit telescope adapter to do astrophotography, mainly deep-sky and I'm wondering if I can use my current Panasonic DMC-FZ50 Type CCD camera body.

The problem is that it has a zoom lens that can't be detached which is why I am worried.


texts: When the sun "enters" a constellation, doesn't it make it impossible to see that constellation?

texts:
I was reading on NASA's website about how the Babylonians came up with dividing the sky into different slices and picked a pattern of stars in each slice to represent it. This ties in to astrological signs, and that being born "under" a sign means that that particular constellation is aligned with the continuation of an imaginary line from earth to sun (and onwards out into space).
But here's what I don't get: when "the sun is in Scorpio", isn't the actual constellation "Scorpius" impossible to see? It would be "behind" the sun, right?
This seems backwards to me, but then again I'm not a Babylonian from 3000 years ago. Am I misunderstanding something, or is this really how it works? You're born "under Aries", yet Aries isn't even visible on the sky at that time?


texts: Are there good animations of the movement of proximate stars over long time periods?

texts:
After reading about Barnard's Star and how it's approaching us 140 km/sec, I'm curious if there are any visualizations of how the nearest stars to Sol will be moving over the next 100k years or so.

I'm amazed that Barnard's Star, currently 6 light years away, will be only 3.8 light years away in about 9,800 years. It's really moving!


texts:
First of all sorry if this question was already asked, but I made a small search here and on google and I couldn't find any answer, so.. Here I am.

I was reading NASA's webpage about the Webb telescope, and I noticed that they are saying that one of the sides will be very hot.

I thought that one of the reasons to put the telescope on the L2 point was to avoid "seeing" the Sun, but apparently the L2 point is too far to have Earth fully cover the Sun.

So.. What is the maximum distance from Earth you have to be in order to have our planet block the Sun? Of course neglecting the effect of the atmosphere, which creates a sort of "halo" even if the Sun is fully covered.

Moreover what will the telescope see towards the sun? Just the star with a very tiny dot in the middle, or a very big black dot (optionally with the moon moving around it), or... What?

Thank you


texts: Why won't the Milky Way colliding with Andromeda affect the Solar System?

texts:
According to Wikipedia

  Such an event would have no adverse effect on the system and the chances of any sort of disturbance to the Sun or planets themselves may be remote.

I'm imagining that this is because the distance between stars is so big, it'd be highly unlikely for any star to actually collide.

But the Oort Cloud is 2 light years in diameter. Given that the average distance between stars in the Milky Way is about 5 light years, that's almost half way. If Alpha Centauri had an Oort Cloud of the same size, we'd already almost be overlapping each other.

Assuming that we're not the only star with an Oort Cloud, isn't there a high probability that the Sun will actually be inside the Oort Cloud of a passing star?

I would imagine this would because the Sun and our planet would be bombarded with lots of comets which have been thrown off their orbits by either the gravity of the Sun or the passing star.


texts: Possibility of certain conditions on a planet

texts:
I want to know if there can be any planet that can fulfill the following conditions and if any planet has been observed with such characteristics:

The planet must not be tidally locked.
The day-night cycle must be similar to our planet or smaller.
The planet must be so close to the star that the light emitted from the star 'burns' (can cause third degree burns to human skin within seconds after exposure).
Temperatures in the shadow cannot exceed 40-50°C (whether prolonged shadow during daytime or conditions during nighttime).
Atmosphere is not necessary.


texts: Doppler Spectroscopy - Finding mass of an exoplanet

texts:
An astronomer, T observed the spectrum of a distant star X. The spectral lines were observed to have tiny blueshifts and redshifts regularly with a period of 1.27 years. T concluded that the star is wobbling slightly due to a planet orbiting it. Let the estimated mass of the planet when

(a) eccentricity of the orbit e = 0 be k*10^23 kg
(b) and for e = 0.6 be x*10^22 kg.
Assume that there is only one major planet and the axis of rotation of the system is perpendicular to the line of sight.

Mass of Star = 23.86163 x 10^30 kg
Time Period = 463.55 days
Shift in wavelength = 1.7627 x 10^-5 nm

You have to find out 'k' and 'x'

I studied about the Doppler Spectroscopy method.
I know that the relation
$$
K = \left(\dfrac{2\pi G}{P_{orb}}\right) ^{1/3}\dfrac{M_p}{(M_{*}+M_p)^{2/3}}\dfrac{1}{\sqrt{1-e^2}}
$$
holds where K = radial velocity half amplitude.
How can I use the doppler shift given into this?
Thanks.


texts: Why can't we see the supermassive black holes supposedly at the center of galaxies?

texts:
Is gravitational lensing responsible for obscuring (figuratively) the supposed supermassive black holes at the center of our own and other galaxies? By "see" I mean relative to the measuring tools we are using. A radio telescope "sees" in radio waves.

In Hubble photos of galaxies, no black holes appear to be present in galaxy centers, at least not visually. How can this be explained?


texts: What is it that distinguishes one atmospheric layer from another?

texts:
The atmosphere of a planetary body (assuming it has an atmosphere) is described as being made up of distinct layers.

For example, Earth, Saturn and Jupiter all have a stratosphere and a troposphere.

What is it that defines where one layer stops and another starts?


texts: Can mass loss via accretion occur on stellar remnants?

texts: We know that normal stars can lose mass to a binary companion. But can this happen to neutron stars and white dwarfs? Let's say a stellar black hole is being orbited by a white dwarf or neutron star. How close would they have to orbit each other for mass loss to occur, if it is possible?


texts: Does Earth changes angular velocity in 24 hours evaluation?

texts: I am trying to detect tidal forces influence on a pendulum. I use a laser interferometer that is able to detect very tiny space changes. The pendulum never stops even after long times quiet and protected from wind by a kind of plastic box. The pendulum is 2,60 m,  long, 10Kg and one of the mirrors of the interferometer is placed in the Iron cable near to the pendulum hanging point. I can see that the top energy of the pendulum changes but I am not sure of the causes.


texts: Is there a website that shows equivalent views through different telescopes?

texts: I'm trying to buy a beginners telescope. I have a Celestron TravelScope, but would like something better. When I look at other models, many users have posted images from their viewfinders. Is there a site that collates all of these, to give an idea of what I can expect from a given scope (e.g. is Mars just a red dot, can I make out Saturn's rings, what's the detail on the moon like, etc)?


texts: Rotational speeds of the material forming planets should increase, not decrease

texts:
We know that planets form as a result of gaseous material revolving around a center of mass that froze to form planets. However shouldn't the angular speed of that matter increase as they go closer to the center to maintain the angular momentum?

Why then the average rotational period of the external planets are faster than that for the terrestrial planets?


texts: How can I make Stellarium match the magnitude of stars visible in London, UK?

texts: Can someone recommend some Stellarium settings to mimic the amount of light pollution, and magnitude of stars that should be visible on a clear night in London, UK?


texts: Spectra from Temperature and Emission Measure

texts: Having the solar flare fluxes on the two bands of GOES (0.05-0.4 nm and 0.1-0.8 nm) I need to calculate the flux between 0.05 and 0.2 nm.


texts: Full picture of our galaxy from the outside

texts: How far a spaceship should travel to take a full picture of our own galaxy from the outside?


texts:
As part of my worldbuilding for a book, I’d like to have a planet with extreme variance in its tides. I want lowest tide  reveal about 30 miles of floodplain, and occasional massive tidal waves. I understand that, in theory, this might be achieved by giving the planet several relatively large moons with different orbital periods, such that sometimes they are spaced out so they balance out the tides (a kind of medium tide), and sometimes they are all together on one side of the planet so that they create a highest / lowest tide.

How stable can such a system be? (The star and other solar system features can be whatever to improve stability, if that is a thing)

Any spitballed numbers for viable orbital periods and tidal patterns?


texts: I encountered a book discussion that defines the Sun's declination as 23.5 degrees (inclination of Earth's equator to the ecliptic) times the sine of the Sun's longitude. Does this make sense? How do i visualize it?


texts:
Many stars are part of constellations. Due to dark matter the stars have velocities which are nearly the same unregardless their radius of orbit around the centre of the Milky Way. Also, it takes about 200 million years to orbit this centre. But nevertheless the angle from which we look at those stars could change a bit during this orbit.
Regarding all these effects, could you say that constellations don't change at all or does it just take a long while?  And of course stars could run out of energy and explode and vanish, so that we don't see them anymore. But is this the only way the constellations could change?


texts:
I have come across a number of star catalogs that list stars by right ascension and declination, along with other data such as magnitude.

Is there a star catalog that lists the 3-D position of stars (e.g. includes distance from the sun, or otherwise specifies a 3-D position)?


texts: Why don't we build an interferometer between Earth and the Moon?

texts:
The Event Horizon Telescope (EHT) does interferometry from an array of telescopes spread all across Earth. The data is locally stored on a hard drive and shipped to a central location, so the telescopes in the array don't need to be connected to each other in real time. Would it be possible to make an EHT-like interferometer with telescopes on the Earth and on the Moon? It seems like such a telescope would have a much higher resolution than the EHT, and would make it possible to observe in wavelengths shorter than radio.

Of course, money is likely an issue here, but is there any technical reason why this wouldn't be feasible?


texts: App Stellarium and hourly wobbling motion of Neptune

texts:
In Stellarium when I zoom in on Neptune to be able to see your daily rotation. I select Neptune, center it (space bar) and then make increments of one solar hour (command +).  I notice that the axis of rotation seems to wobble (just focus on the big dark spot to notice that), but the precession period of Neptune's axis of rotation is hundreds of years, not hours. How is this possible?. I think it may be due to the combination of the daily rotational movement of us (the Earth) and Neptune, but I cannot get a clear geometric representation of this. Does anyone know why this phenomenon happens?
Thank you.
I have: Stellarium 0.20.4 for Mac OS.


texts:
I want to ask a question about the Three-torus model of the universe, as described in wikipedia:

https://en.wikipedia.org/wiki/Three-torus_model_of_the_universe

"is a proposed model describing the shape of the universe as a three-dimensional torus."

I'm not sure i understand the maths of three-dimensional tori, but I want to ask:

Does this model posit that the shape of the universe is that of a ring doughnut? If so, is the universe contained inside the ring doughnut, or is it supposed to exist only on the surface?

Or does the model say that the universe has the shape of a higher dimensional version of a ring doughnut? If so, does it exist inside the higher dimensional ring doughnut, or on its surface?


texts: I am going through the theory of saha equation but I am not able to understand the concept of ionisation state? What are ionization states actually?


texts: What are these faint streaks on the moon?

texts:
I was poking around this Gigapan image of the moon's north polar region, when I noticed this area:

What is the source of these light streaks?

For position reference, here's a zoomed-out shot (I couldn't easily get coordinates from the Gigapan thing):


texts: Is the LSST (Vera C. Rubin) telescope going to change position every 20 or 35 seconds?

texts:
This is a very particular question, I realize, but I am still curious...
The Vera Rubin telescope is supposed to take a 15-second exposure, then take 5 seconds to move to its next spot...
But I also read that it will take two consecutive 15-second exposures at each stopping point, then spend 5 seconds moving to its next position...
On Wikipedia, in the middle of the article on the Vera C. Rubin LSST Telescope, two consecutive paragraphs contradict each other, the first saying it will spend only 15 seconds at each spot before moving on, and the second saying it will take two consecutive 15- second exposures at each point.....


texts: The magnetic field of a magnetar is so strong that it would stretch people's atoms apart. This, would obviously, result in death. But how spaghettification refers to death by extreme tides, and for the most part, refers to death by black hole, what field-specific term would best refer to death by extreme magnetic fields, eg. from a magnetar?


texts: Accuracy of Laplace Method for determining orbital elements

texts:
Recently I had to implement Laplace method and apply it to 3 observations of Mars (10 days in between two observations). The results were pretty good, with discrepancies with real data well below 5% in most of the orbital elements.

I supposed that Laplace method was very accurate, so I tried to apply it to an asteroid (CASLEO, 5387). The 3 observations were also 10-days spaced but now the results widely diverge from the real orbital elements.

I expected it to be less accurate than in the Mars case since the observations covered less fraction of the orbit, but some of the elements were quite far from expected.

So my question is: What is the accuracy of the Laplace Method? Is it really that sensitive to the distance?


texts:
I'm currently stuck on a problem.
I have the RA and DEC for JD2000:

RA = 17 57 48.49980

DEC = + 04 41 36.1114

I want to calculate the RA and DEC for 01.01.2017, how can I do it?
The solution might be easy but I'm totally stuck right now.

Thanks in advance.


texts: What are recent discoveries with Earth-based radar telescopes?

texts:
The Wikipedia article on radar astronomy lists a few important discoveries using radar technology in general, i.e. also using space-crafts:

The following is a list of planetary bodies that have been observed by this means:

Mercury - Improved value for the distance from the earth observed (GR test). Rotational period, libration, surface mapping, esp. of polar regions.
Venus - first radar detection in 1961. Rotation period, gross surface properties. The Magellan mission mapped the entire planet using a radar altimeter.
Earth - numerous airborne and spacecraft radars have mapped the entire planet [...]
Mars - Mapping of surface roughness from Arecibo Observatory. The Mars Express mission carries a ground-penetrating radar.
Jupiter System - Galilean satellites
Saturn System - Rings and Titan from Arecibo Observatory, mapping of Titan's surface and observations of other moons from the Cassini spacecraft.

I am interested in recent discoveries (and scientific references) with Earth-based radar observatories such as the Arecibo Observatory, the Goldstone Deep Space Communications Complex, RT-70, Pluton, Deep Space Network and any other one I missed.
Here "discovery" is meant in the wider sense, i.e. the gain of scientific knowledge. The discovery of a new object would be such a discovery, but the mapping of a known object would also be a discovery.
References

What will succeed the Arecibo Observatory?
How did Arecibo detect methane lakes on Titan, and image Saturn's rings?


texts: Could Dark Energy be a "Cosmic Gravity Background"

texts: Could dark energy (the accelerating expansion of the universe) be the result of gravity from the dense past universe "pulling" outward from all directions? The universe theoretically started out extremely dense and hot, and we can see the light reaching us as the cosmic microwave background. I wonder if that same region and denser portions beyond are exerting a gravitational effect that is stretching out everything within toward it, effectively being a cosmic gravity background. If so, is this the explanation for dark energy?


texts: How distances are measured in the Solar System, in the Galaxy, and in the Universe?

texts:
Distances in our universe can be calculated in several ways:

Stellar Parallax
Standard Candles
Redshift

How are these methods linked, and how they are actually used to calculate   distances are measured in the Solar System, in the Galaxy, and in the Universe?


texts: What were the odds that ʻOumuamua passed so close to the Sun?

texts:
ʻOumuamua passed its closest point at about 0.25 UA from the Sun.

Has anyone researched what the odds were that the first ever spotted interstellar object passed so close to the Sun?

Being the interstellar space almost totally empty, I would intuitively suppose that such odds would be extremely low.


texts:
Comets orbiting the sun have a tail that points away from the Sun, due to the solar wind.

(Strictly speaking, a comet can have both a type I tail - ion tail, and a type II tail - a dust tail. This question is about the type II tail - the dust tail).

Now I understand that some of the source of the dust is volatile materials in the comet oxidising due to the comet's proximity to the sun. But surely some of these comets have been orbiting for hundreds of thousands of years (if not millions). Why do they still have unspent fuel? Why are they still dusty? Hasn't all the dust blown off by now?

My question is: Why do comets appear to have a continuous supply of dust?


texts: Is it possible for a moon to only be out at night?

texts:
(For the sake of this question, let's assume that all other factors are malleable.)

Essentially creating a permanent lunar eclipse, could a moon orbit around a planet so slowly that it is always opposite the sun from its planet?